When, for example, parts are assembled using a robot arm, a force sensor is mounted to a wrist portion of the robot arm, detects a force or a moment component that is generated during an assembly work, and controls an orientation of a hand portion. A force sensor that makes use of a method of magnetically detecting a displacement of an action portion is discussed in Japanese Patent Laid-Open No. 2004-325328 (Patent Document 1).
A related structure that is discussed in Japanese Patent Laid-Open No. 2004-325328 is shown in FIG. 5. In the related structure, a permanent magnet 14 embedded in a resilient member and four magneto-electric transducers 15a to 15d opposing magnetic poles of the permanent magnet are disposed. When an action force is applied to the resilient member, the permanent magnet 14 is displaced, and the magneto-electric transducers 15a to 15d, such as Hall elements, detect a change in a magnetic field that is generated thereby. On the basis of the detected change in the magnetic field, it is possible to detect force components in an X-axis direction, a Y-axis direction, and a Z-axis direction.
In Japanese Patent Laid-Open No. 2004-325328, a permanent magnet is used for a magnetic flux generating source. In general, an absolute value of the strength of a magnetic field that is generated by a permanent magnet is changed by the temperature of the permanent magnet (that is, the absolute value has a predetermined temperature coefficient). When the temperature of the permanent magnet is increased due to heat generated from a circuit board and heat received from the outside, such as a motor disposed in the vicinity thereof, the strength of the magnetic field is reduced. Therefore, an apparatus used in the magnetic field generated by the permanent magnet needs to compensate for the change in the magnetic field caused by its temperature. The magnetic field changes due to changes with time of the permanent magnet in addition to the influences of, for example, the temperature thereof.
In contrast, in Japanese Patent Laid-Open No. 2005-321592 (Patent Document 2), a method of performing temperature compensation using a temperature compensation circuit that is provided with a temperature sensor is used.
A related structure discussed in Japanese Patent Laid-Open No. 2005-321592 is shown. FIG. 6A shows a Hall element 16 provided at a position opposite to a permanent magnet 14. FIG. 6B shows a temperature compensation circuit connected to a constant voltage source (not shown) provided adjacent to the Hall element.
When an output voltage from the Hall element to a selector is reduced by a temperature rise, a negative temperature characteristic of the temperature sensor provided in the temperature compensation circuit provided adjacent to the Hall element causes a current value output to the Hall element from the temperature compensation circuit to increase in proportion to the temperature rise.
By outputting the increased current to the Hall element, the output voltage from the Hall element 16 to the selector that has been reduced by the temperature rise is increased.
However, in the related example discussed in Patent Document 2, temperature compensation is performed on the output of the Hall element that receives a magnetic flux. Therefore, when there is a difference between the heat capacities of the permanent magnet and the temperature sensor (specific heat) or a localized temperature rise (for example, the temperature of the Hall element changes whereas the temperature of the permanent magnet 14 itself is increased), it is difficult for the temperature sensor to precisely measure the average temperature of the permanent magnet. In addition, the temperature compensation circuit needs to be provided separately from a sensor of, for example, the permanent magnet and the Hall element, thereby increasing costs and size.
The present invention is carried out to overcome the aforementioned problems, and provides a compact magnetic force sensor that can precisely detect and correct variations in a magnetic field generated by a magnetic flux generating source.